It is known in the prior art to form emission-enhancing coatings on the surfaces of electron emitter structures of field emission devices. These prior art coatings are employed to improve the emission current characteristics of the field emission device. Typically, the electron emitter structures are Spindt-tip structures made from molybdenum, and the emission-enhancing coating is a metal that is selected for its low work function, which is less than that of the molybdenum. The surface work function of molybdenum is about 4.6 eV. Processes for forming electron emitter structures, such as Spindt tips, from molybdenum are well known in the art.
Prior art emission-enhancing coatings are known to be made from a pure metal selected from the following: sodium, calcium, barium, cesium, titanium, zirconium, hafnium, platinum, silver, and gold. Also known are emission-enhancing coatings made from the carbides of hafnium and zirconium. These prior art coatings are known to improve the emission current characteristics of field emission electron emitters.
However, these prior art coatings suffer from several disadvantages. For example, many of the prior art coatings, such as those made from the alkali and alkaline earth metals, are extremely reactive with respect to certain gaseous species, such as oxygen-containing species, that are present within the field emission device. Many of the prior art coatings are susceptible to oxidation during the operation of the device, resulting in emission instabilities. The alkali and alkaline earth metals also have high surface diffusion coefficients. Thus, subsequent to their deposition, these species do not remain stationary on the surface of the electron emitter structure. These characteristics of high reactivity and surface mobility result in emission current instabilities, poor device lifetime, and stringent vacuum requirements.
It is also known in the art to coat electron emitters with films made from diamond-like carbon. This prior art coating is similarly employed for the purpose of reducing the work function of the surface of the electron emitters.
When the electron emitter structures are made from a metal and do not have an emission-enhancing coating formed thereon, the surfaces of the electron emitter structures react with oxygen-containing, gaseous species contained within the device, thereby transforming the surfaces of the electron emitter structures to an oxide of the metal. Typically, water vapor, oxygen, carbon dioxide, and carbon monoxide are present in amounts sufficient to cause appreciable oxidation of the molybdenum emitter surfaces during the operation of the device. The changing characteristics of the surfaces of the electron emitter structures result in emission current instabilities. Further, molybdenum oxide, the oxide of the metal from which electron emitter structures are typically made, has a work function that is greater than that of pure molybdenum, resulting in electron emission characteristics that are inferior to those of the pure molybdenum surface.
Accordingly, there exists a need for an improved field emission device having electron emitters that are resistant to oxidation during the operation of the device and that have surface work functions that are less than or equal to that of the metal from which the electron emitter structures are made.